RUNOFF LENGTH OVER TERRICOLOUS LICHEN CRUSTS DEPENDING ON RAINFALL
INTENSITY AND ANTECEDENT SOIL MOISTURE
Roberto Lázaro-Suau1, Emiliano Pegoraro1 and Jane Bevan2.
1Estación Experimental de Zonas Áridas (CSIC); General Segura, 1; 04001 – Almería, Spain
(Arid Zones Experimental Station, of the Spanish national Upper Council for Scientific Research)
2Department of Geography & Develop. University of Chester, Parkgate Road Chester CH1 4BJ, UK
R. Lázaro: lazaro@eeza.csic.es; E. Pegoraro: emi@eeza.csic.es; J. Bevan: j.bevan@chester.ac.uk
Knowledge of the hydrological behaviour of the different pure soil surface types at small plot
scale is insufficient to directly understand the hydrological behaviour at slope or small catchment
scales (Cantón et al., 2002). To up-scale the results from small plots to slope we need to know the
length of the runoff, that is to say, the distance covered by the water flows in specific circumstances.
This length depends on surface characteristics, especially including vegetation density, features and
distribution, precipitation intensity and antecedent soil moisture. There is no data on runoff length to be
found in the literature, and therefore, we aimed to experimentally find them in the natural environment.
Firstly, we selected an area in badlands, because it is so rich in landforms in a small space that a 4-mlong plot is practically equivalent to a (small) slope, and 4 m is small enough for artificial rainfall
experiments. This is important because a slope is a functional unit, whereas a plot is not. So, in this
area, an experiment over 4 m can represent a change in organization level, making it possible to upscale hydrological responses. To start with, we selected a certain class of surfaces, the biological soil
crusts, because they are relatively simple and homogeneous. These crusts are frequent, at several
spatial scales, in inhospitable or relatively inhospitable sites (Budel, 2001), and play an important
ecological role, affecting hydrology and erosion (Solé et al. 1997; Yair, 2001), physical-chemical soil
properties (Belnap and Gardner, 1993) and recruitment of vascular plants (Harper and St.Clair, 1985).
It seems to be widely accepted that living crusts increase surface runoff and, simultaneously,
decrease water erosion; although some discussion exists, probably due to the diversity of living crusts
and to the effect of differences in particle size in the upper soil horizon. In the Iberian semiarid
Southeast, these crusts are dominated by terricolous lichens, which can locally cover large areas,
constituting a natural mechanism for stabilisation and helping to build up vegetation patterns by
occupying places with less available water, while providing the vascular plants clustered downslope
with extra water by increasing runoff.
The objectives of this work were (i) to construct a portable rainfall simulator able to generate 3
different stable, repeatable rainfall intensities, homogeneously distributed over an area of 2 m x 6 m,
and no more than 2 m high, (ii) observe the relationship between the amount of runoff and slope
length, (iii) find out how rainfall intensity and antecedent soil moisture affect the distance covered by
runoff. Our initial hypotheses are, (i) that runoff covers a distance specific to each combination of
given factors and then re-infiltrates, therefore, runoff increases with the length of slope considered
only up to a certain point, after which it remains more or less constant (for homogeneous slopes with
no change in angle); (ii) that runoff increases with rainfall intensity and with antecedent soil moisture;
and (iii) that, runoff starts after a threshold of cumulated rainfall has been reached, then increases fast
and afterwards stabilizes in a more or less constant (or oscillating) flow.
We erected a rainfall simulator with the required characteristics, able to generate precipitation
at 40, 48 and 63 mm/h, using a series of 12 Hardy® nozzles, models 15, 21 and 25, respectively,
which sprinkle water in fans, wetting long, practically rectangular areas of approximately of 0.5 m x
1.9 m. In the Tabernas Desert badlands (SE Spain), we selected typical areas covered by
homogeneous biological soil crusts, representative of two of the three main lichen communities, with
very little vascular plant cover. This experiment was not possible in the third community, because the
site is too steep and the cover of vascular plants is significant. Area S has a lichen crust dominated by
white lichens with large, thick thalli, such as Squamarina lentigera, Diploschistes diacapsis, Buellia
zoharyi and others. In Area U, undifferentiated brown living crust, rich in cyanobacteria, algae and
fungus hyphae, dominates, although there is an important diversity of generally smaller terricolous
lichens, such as Endocarpon pusillum, Fulsgensia sps., Placynthium nigrum, Collema sps, and others.
The experiment includes 3 rainfall intensities, 4 slope lengths (1, 2, 3 and 4 m), and 4 replicates of
each combination, requiring a total of 48 0.4-m-wide bounded plots in each community. The plots were
arranged in blocks of 4. Every block included one plot of each length, in random order, and the blocks
were grouped in 6 1.75-m x 5.5-m pairs, so that artificial rainfall could be sprinkled simultaneously
over 8 plots, which was necessary for logistic reasons. We had previously confirmed that sub-surface
water flows from the plots in the upper block do not affect the lower plots, and that 20 minutes is long
enough for runoff over initially dry soil to become stable. Three consecutive experiments were
performed in each double block in eight plots: 20 minutes of rainfall on dry soil (after a very dry
summer) followed by a 20-minute pause to allow the soil moisture to abate, then 15 minutes of rainfall
and a 5-minute pause (the minimum to prepare the
third experiment, to keep soil moisture from going
down) and 10 minutes of rainfall. The second and
third experiments are done with progressively
increasing antecedent soil moisture. Two double
blocks are used for each rainfall intensity and we
attempted to distribute the slope angles
approximately equally among the intensities, and to
put the plots in areas with typical lichen cover
values. During the experiments, runoff leaving each
plot was measured at several times.
In the photograph on left, the rainfall simulator
over a double block of bounded plots.
The first results show that: a) For all rainfall intensities, the runoff flow rate (that is, the amount
of runoff water per unit of time) increases with the slope length up to approximately 3 meters and then
stabilizes or decreases, and this pattern seems to be independent of soil moisture; b) Runoff flow rate
increases with rain intensity, however, in the undifferentiated crust it increases only from the medium
intensity (except in the case of the maximum length, in which the increment in runoff is linear for all
intensities), and this pattern is also the same for all antecedent soil moisture values; c) Runoff flow
rate increases with the antecedent soil moisture in all cases. But, although these trends seem clear
and the patterns are repeated for all the rainfall intensities as well as for the different antecedent soil
moisture levels, differences in slope length, rain intensity and soil moisture often do not produce
significantly different effects on runoff. The runoff flow rate seems to pulse over time, probably due to
the roughness of the surface and, on the other hand, for several reasons, runoff starts at different
times in plots of the same length. Both things cause wide dispersion of runoff values for a given time
when data from the four replicas are combined, which would explain the lack of significance of a good
part of the results. However, there are interesting differences between the hydrological responses of
the two biological soil crusts. Runoff in the community dominated by the undifferentiated crust is
higher than in the Squamarina community. This result was a little unexpected, as the thick
Squamarina lichen thalli suggest that this community is relatively more waterproof. But, all things
considered, this result is consistent with the crust dynamics and functioning, because the community
dominated by undifferentiated crust is a pioneer and primo-colonizer, according to unpublished own
data, and this ability to export water would allow it to diminish the available water within its space,
probably under the minimum threshold for the development of vascular plants, with which living crust
cannot compete for light.
References
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on the overall runoff of the Tabernas badlands (SE Spain): field data and model approaches.
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Key words
Distance covered by runoff, biological soil crusts, semiarid, rainfall simulation, Tabernas Desert